Chronic fatigue syndrome (CFS) is a debilitating multi-symptom disorder characterized by unexplained and prolonged fatigue, whose diagnosis is currently based on a relatively broad clinical case definition. Consequently, the pool of CFS patients included in clinical studies of the illness is greatly heterogeneous - a fact that might have impeded research progress to date. A major step forward in understanding the pathophysiology of CFS would involve reducing this heterogeneity by identifying one or more subgroups of patients with different pathophysiological causes of their illness, and then selecting one of these subgroups for inclusion into research studies. Over the past few years, we and others have provided substantial data supporting the existence of a subgroup of patients with a neurobiological cause for their illness, based on stratifying the sample according to the absence or presence of comorbid Axis I psychopathology (CFS-no psych or CFS-NP and CFS-psych or CFS-P, respectively). Compared to CFS-P patients, the CFS-NP patients had more cognitive dysfunction, a higher rate of abnormal cerebrospinal fluid (CSF) findings, lower regional cerebral blood flow (rCBF), and higher ventricular CSF lactate values. A further complication and limitation of these studies is that each had investigated only one brain-related variable, whose utility in separating CFS patients into subgroups was limited. The purpose of the present Exploratory/Developmental Research Grant (R21) proposal is to rigorously assess and confirm whether patients in the CFS- NP group have consistent abnormalities across several different neuropathological variables - an outcome that would be expected if this group, in fact, does have distinct neurobiological underpinnings. Specifically, in the same subjects, we will (a) assess cognitive function using objective neuropsychological testing;(b) conduct biochemical analysis of spinal fluid samples obtained by lumbar puncture;and (c) measure rCBF and ventricular lactate using magnetic resonance imaging and spectroscopy, respectively, in CFS-P and CFS-NP patients. This will allow us to test the hypothesis that CFS-NP patients have more abnormalities in these outcome variables than CFS-P patients. Our second Aim will use the results from the first Aim in a cluster analysis to attempt objective, data-driven classification of the CFS subjects into subtypes, and then compare the resulting subgroups based on membership into CFS-NP or CFS-P groups. This aim will test the hypothesis that the results of the cluster analysis will identify a group with abnormalities across the multiple brain-based variables studied, and this group will be constituted of significantly more CFS-NP patients than in other groups. PUBLIC HEALTH RELEVANCE: Chronic fatigue syndrome (CFS) is a medically unexplained debilitating disease that is diagnosed by the presence of a set of predefined symptoms. Because there are no validated diagnostic tests for the illness, progress in understanding its causes has been slow. From a series of studies by our group and others there has emerged strong preliminary evidence that supports the existence of a subgroup of CFS patients whose illness appears to be due to specific biological abnormalities in the brain. The purpose of this Exploratory/Developmental grant proposal is to conduct carefully controlled studies that will seek to confirm the existence of such a subgroup of CFS patients. If successfully completed, this study could validate an approach for selecting more homogeneous CFS patient populations in future research studies to enable them to focus better on understanding the exact cause of CFS and developing effective treatments for the illness.